The design of covalent diabodies is based on the single chain Fv construct (scFv) (Holliger et al. (1993) “‘Diabodies’: Small Bivalent And Bispecific Antibody Fragments,” Proc. Natl. Acad. Sci. USA 90:6444-6448; herein incorporated by reference in its entirety). In an intact, unmodified IgG, the VL and VH domains are located on separate polypeptide chains, i.e., the light chain and the heavy chain, respectively. Interaction of an antibody light chain and an antibody heavy chain and, in particular, interaction of VL and VH domains forms one of the epitope binding sites of the antibody. In contrast, the scFv construct comprises a VL and VH domain of an antibody contained in a single polypeptide chain wherein the domains are separated by a flexible linker of sufficient length to allow self-assembly of the two domains into a functional epitope binding site. Where self assembly of the is impossible due to a linker of insufficient length (less than about 12 amino acid residues), two of the scFv constructs interact with each other to form a bivalent molecule, the VL of one chain associating with the VH of the other (reviewed in Marvin et al. (2005) “Recombinant Approaches To IgG-Like Bispecific Antibodies,” Acta Pharmacol. Sin. 26:649-658). Moreover, addition of a cysteine residue to the c-terminus of the construct has been show to allow disulfide bonding of the polypeptide chains, stabilizing the resulting dimer without interfering with the binding characteristics of the bivalent molecule (see e.g., Olafsen et al. (2004) “Covalent Disulfide-Linked Anti-CEA Diabody Allows Site-Specific Conjugation And Radiolabeling For Tumor Targeting Applications,” Prot. Engr. Des. Sel. 17:21-27). Further, where VL and VH domains of differing specificity are selected, not only a bivalent, but also a bispecific molecule may be constructed.
Bivalent diabodies have wide ranging applications including therapy and immunodiagnosis. Bivalency allows for great flexibility in the design and engineering of diabody in various applications, providing enhanced avidity to multimeric antigens, the cross-linking of differing antigens, and directed targeting to specific cell types relying on the presence of both target antigens. Due to their increased valency, low dissociation rates and rapid clearance from the circulation (for diabodies of small size, at or below ˜50 kDa), diabody molecules known in the art have also shown particular use in the filed of tumor imaging (Fitzgerald et al. (1997) “Improved Tumour Targeting By Disulphide Stabilized Diabodies Expressed In Pichia pastoris,” Protein Eng. 10:1221). Of particular importance is the cross linking of differing cells, for example the cross linking of cytotoxic T cells to tumor cells (Staerz et al. (1985) “Hybrid Antibodies Can Target Sites For Attack By T Cells,” Nature 314:628-631, and Holliger et al. (1996) “Specific Killing Of Lymphoma Cells By Cytotoxic T-Cells Mediated By A Bispecific Diabody,” Protein Eng. 9:299-305). Diabody epitope binding domains may also be directed to a surface determinant of any immune effector cell such as CD3, CD16, CD32, or CD64, which are expressed on T lymphocytes, natural killer (NK) cells or other mononuclear cells. In many studies, diabody binding to effector cell determinants, e.g., Fcγ receptors (FcγR), was also found to activate the effector cell (Holliger et al. (1996) “Specific Killing Of Lymphoma Cells By Cytotoxic T-Cells Mediated By A Bispecific Diabody,” Protein Eng. 9:299-305; Holliger et al. (1999) “Carcinoembryonic Antigen (CEA)-Specific T-cell Activation In Colon Carcinoma Induced By Anti-CD3 x Anti-CEA Bispecific Diabodies And B7 x Anti-CEA Bispecific Fusion Proteins,” Cancer Res. 59:2909-2916). Normally, effector cell activation is triggered by the binding of an antigen bound antibody to an effector cell via Fc-FcγR interaction; thus, in this regard, diabody molecules of the invention may exhibit Ig-like functionality independent of whether they comprise an Fc domain (e.g., as assayed in any efferctor function assay known in the art or exemplified herein (e.g., ADCC assay)). By cross-linking tumor and effector cells, the diabody not only brings the effector cell within the proximity of the tumor cells but leads to effective tumor killing (see e.g., Cao et al. (2003) “Bispecific Antibody Conjugates In Therapeutics,” Adv. Drug. Deliv. Rev. 55:171-197, hereby incorporated by reference herein in its entirety).
2.1 Effector Cell Receptors and their Roles in the Immune System
In traditional immune function the interaction of antibody-antigen complexes with cells of the immune system results in a wide array of responses, ranging from effector functions such as antibody-dependent cytotoxicity, mast cell degranulation, and phagocytosis to immunomodulatory signals such as regulating lymphocyte proliferation and antibody secretion. All these interactions are initiated through the binding of the Fc domain of antibodies or immune complexes to specialized cell surface receptors on hematopoietic cells. The diversity of cellular responses triggered by antibodies and immune complexes results from the structural heterogeneity of Fc receptors. Fc receptors share structurally related an antigen binding domains which presumably mediate intracellular signaling.
The Fcγ receptors, members of the immunoglobulin gene superfamily of proteins, are surface glycoproteins that can bind the Fcγ portion of immunoglobulin molecules. Each member of the family recognizes immunoglobulins of one or more isotypes through a recognition domain on the alpha chain of the Fey receptor. Fcγ receptors are defined by their specificity for immunoglobulin subtypes. Fcγ receptors for IgG are referred to as FcγR, for IgE as FcεR, and for IgA as FcαR. Different accessory cells bear Fcγ receptors for antibodies of different isotype, and the isotype of the antibody determines which accessory cells will be engaged in a given response (reviewed by Ravetch J. V. et al. (1991) “Fc Receptors,” Annu. Rev. Immunol. 9: 457-92; Gerber J. S. et al. (2001) “Stimulatory And Inhibitory Signals Originating From The Macrophage Fcgamma Receptors,” Microbes and Infection, 3: 131-139; Billadeau D. D. et al. (2002), “ITAMs Versus ITIMs: Striking A Balance During Cell Regulation,” The Journal of Clinical Investigation, 2(109): 161-1681; Ravetch J. V. et al. (2000) “Immune Inhibitory Receptors,” Science, 290: 84-89; Ravetch J. V. et al., (2001) “IgG Fc Receptors,” Annu. Rev. Immunol. 19:275-90; Ravetch J. V. (1994) “Fc Receptors: Rubor Redux,” Cell, 78(4): 553-60). The different Fcγ receptors, the cells that express them, and their isotype specificity is summarized in Table 1 (adapted from Immunobiology: The Immune System in Health and Disease, 4th ed. 1999, Elsevier Science Ltd/Garland Publishing, New York).
Fcγ Receptors
Each member of this family is an integral membrane glycoprotein, possessing extracellular domains related to a C2-set of immunoglobulin-related domains, a single membrane spanning domain and an intracytoplasmic domain of variable length. There are three known FcγRs, designated FcγRI(CD64), FcγRII(CD32), and FcγRIII(CD16). The three receptors are encoded by distinct genes; however, the extensive homology between the three family members suggest they arose from a common progenitor perhaps by gene duplication.
FcγRII(CD32)
FcγRII proteins are 40 kDa integral membrane glycoproteins which bind only the complexed IgG due to a low affinity for monomeric Ig (106 M−1). This receptor is the most widely expressed FcγR, present on all hematopoietic cells, including monocytes, macrophages, B cells, NK cells, neutrophils, mast cells, and platelets. FcγRII has only two immunoglobulin-like regions in its immunoglobulin binding chain and hence a much lower affinity for IgG than FcγRI. There are three human FcγRII genes (FcγRII-A, FcγRII-B, FcγRII-C), all of which bind IgG in aggregates or immune complexes.
Distinct differences within the cytoplasmic domains of FcγRII-A and FcγRII-B create two functionally heterogenous responses to receptor ligation. The fundamental difference is that the A isoform initiates intracellular signaling leading to cell activation such as phagocytosis and respiratory burst, whereas the β isoform initiates inhibitory signals, e.g., inhibiting B-cell activation.
FcγRIII (CD16)
Due to heterogeneity within this class, the size of FcγRIII ranges between 40 and 80 kDa in mouse and man. Two human genes encode two transcripts, FcγRIIIA, an integral membrane glycoprotein, and FcγRIIIB, a glycosylphosphatidyl-inositol (GPI)-linked version. One murine gene encodes an FcγRIII homologous to the membrane spanning human FcγRIIIA. The FcγRIII shares structural characteristics with each of the other two FcγRs. Like FcγRII, FcγRIII binds IgG with low affinity and contains the corresponding two extracellular Ig-like domains. FcγRIIIA is expressed in macrophages, mast cells and is the lone FcγR in NK cells. The GPI-linked FcγRIIIB is currently known to be expressed only in human neutrophils.
Signaling Through FcγRs
Both activating and inhibitory signals are transduced through the FcγRs following ligation. These diametrically opposing functions result from structural differences among the different receptor isoforms. Two distinct domains within the cytoplasmic signaling domains of the receptor called immunoreceptor tyrosine based activation motifs (ITAMs) or immunoreceptor tyrosine based inhibitory motifs (ITIMS) account for the different responses. The recruitment of different cytoplasmic enzymes to these structures dictates the outcome of the FcγR-mediated cellular responses. ITAM-containing FcγR complexes include FcγRI, FcγRIIA, FcγRIIIA, whereas ITIM-containing complexes only include FcγRIIB.
Human neutrophils express the FcγRIIA gene. FcγRIIA clustering via immune complexes or specific antibody cross-linking serves to aggregate ITAMs along with receptor-associated kinases which facilitate ITAM phosphorylation. ITAM phosphorylation serves as a docking site for Syk kinase, activation of which results in activation of downstream substrates (e.g., PI3K). Cellular activation leads to release of proinflammatory mediators.
The FcγRIIB gene is expressed on B lymphocytes; its extracellular domain is 96% identical to FcγRIIA and binds IgG complexes in an indistinguishable manner. The presence of an ITIM in the cytoplasmic domain of FcγRIIB defines this inhibitory subclass of FcγR. Recently the molecular basis of this inhibition was established. When co-ligated along with an activating FcγR, the ITIM in FcγRIIB becomes phosphorylated and attracts the SH2 domain of the inosital polyphosphate 5′-phosphatase (SHIP), which hydrolyzes phosphoinositol messengers released as a consequence of ITAM-containing FcγR-mediated tyrosine kinase activation, consequently preventing the influx of intracellular Ca++. Thus crosslinking of FcγRIIB dampens the activating response to FcγR ligation and inhibits cellular responsiveness. B cell activation, B cell proliferation and antibody secretion is thus aborted.
TABLE 1Receptors for the Fc Regions of Immunoglobulin IsotypesReceptorBindingCell TypeEffect of LigationFcγRIIgG1MacrophagesUptake Stimulation(CD64)108 M−1NeutrophilsActivation of respiratoryEosinophilsburstDendritic cellsInduction of killingFcγRII-AIgG1MacrophagesUptake Granule Release(CD32)2 × 106 M−1NeutrophilsEosinophilsDendritic cellsPlateletsLangerhan cellsFcγRII-B2IgG1MacrophagesUptake Inhibition of(CD32)2 × 106 M−1NeutrophilsStimulationEosinophilsFcγRII-B1IgG1B cellsNo Uptake(CD32)2 × 106 M−1Mast cellsInhibition of StimulationFcγRIIIIgG1NK cellsInduction of Killing(CD16)5 × 105 M−1EosinophilMacrophagesNeutrophilsMast CellsFcεRIIgEMast cellsSecretion of granules1010 M−1EosinophilBasophilsFcαRIIgA1, IgA2MacrophagesUptake Induction of(CD89)107 M−1NeutrophilsKillingEosinophils